Still Curious After All These Years
Professor:
What’s the Michaelson Morley expiriment?
ChatGPT:
The Michelson-Morley experiment, conducted in 1887 by Albert A. Michelson and Edward W. Morley, is one of the most important and famous experiments in the history of physics. Its main objective was to detect the presence of the “luminiferous aether,” a hypothetical medium through which light waves were thought to propagate, similar to how sound waves travel through air.
At the time, it was believed that if the aether existed, the Earth moving through it would create an “aether wind” effect. Michelson and Morley designed their experiment to measure the speed of light in different directions, in the expectation that the speed of light would vary slightly depending on the direction of the Earth’s movement through the aether. They used an interferometer, a device that splits a beam of light into two paths that travel in perpendicular directions, reflect back, and then recombine. Any difference in the speed of light along the two paths caused by the Earth’s motion through the aether would result in a slight shift in the interference pattern of the recombined beams.
Contrary to the expectations of the time, the Michelson-Morley experiment found no significant difference in the speed of light in different directions. This result was initially puzzling and somewhat controversial, but it eventually led to a profound shift in our understanding of physics. The failure to detect an aether wind was one of the key pieces of evidence that helped Albert Einstein develop his theory of Special Relativity. Einstein’s theory proposed that the speed of light is constant in all inertial frames of reference and does not depend on the motion of the source or observer, effectively eliminating the need for the concept of the luminiferous aether.
Professor:
Thought experiment: What if instead of blasting waves of light in different directions, M&M had blasted waves of sound in opposite directions? Would you expect the beam of sound traveling with the Earth’s rotation to be faster than the other beam of sound?
ChatGPT:
In a thought experiment where Michelson and Morley had used sound waves instead of light waves, the principles and outcomes would indeed be different, owing to the fundamental differences between how sound and light propagate.
Sound waves require a medium (such as air, water, or solid materials) to travel, and their speed is influenced by the properties of that medium, including its temperature, density, and state (solid, liquid, or gas). If Michelson and Morley had conducted their experiment with sound waves, they would have been measuring the speed of sound through the air in different directions relative to the Earth’s rotation.
In this scenario, if one sound wave was traveling in the same direction as the Earth’s rotation (let’s call this the “with the rotation” direction) and another wave was traveling in the opposite direction (the “against the rotation” direction), there would indeed be a difference in the observed speeds of the two sound waves relative to their source and detector on Earth. This difference would be due to the motion of the air (and thus the medium carrying the sound) relative to the ground, as the Earth’s rotation affects the movement of the atmosphere due to various factors including but not limited to the Coriolis force.
The sound wave traveling “with the rotation” might be expected to travel faster relative to the ground-based observer than the sound wave traveling “against the rotation,” because the medium itself (air) would be moving (albeit very slightly) with the Earth’s rotation. This is similar in principle to how an airplane traveling with the jet stream can cover a distance faster than one traveling against it, because the airplane’s speed is relative to the air through which it is moving, not just relative to the ground.
However, it’s important to note that the scale of this effect for sound waves in the atmosphere due to Earth’s rotation would likely be very small and influenced by many variables, such as wind speed and direction, temperature, and atmospheric pressure. The experiment’s outcome would demonstrate the principle that the speed of sound is dependent on the motion of the medium through which it travels, in stark contrast to light, whose speed remains constant regardless of the motion of the source or observer in a vacuum, as demonstrated by the actual Michelson-Morley experiment and explained by Einstein’s theory of special relativity.
Professor:
So for sound, you agree that the medium (air) generally moves with the earth so you wouldn’t expect it to affect the speed of sound waves much. But it does have small effects (like Coriolis) that could cause a small effect. Is that a correct summary of your position?
ChatGPT:
Yes, that’s a correct summary. The medium through which sound waves travel, such as air, generally moves with the Earth due to the atmosphere being bound by gravity and the frictional forces between the air and the Earth’s surface. Thus, for most practical purposes, we wouldn’t expect the Earth’s rotation to significantly affect the speed of sound waves because the medium itself is rotating with the Earth.
However, as you noted, there are small effects, such as the Coriolis effect, which could influence the movement of air masses on a large scale and hence could potentially have a slight effect on the propagation of sound waves over very long distances. These effects are relatively minor compared to the overall speed of sound through air and would likely be challenging to measure outside of highly controlled or extreme conditions. The primary factors affecting the speed of sound in air remain the temperature, humidity, and atmospheric pressure, rather than the Earth’s rotation.
Professor:
OK. Now imagine it’s 1887 minus one. To the best of our knowledge light waves travel through “luminiferous aether” and M&M are about to prove it. What -89-year old me wonders is why everybody expects luminiferous aether to be fixed in space with the Earth slicing through it harmlessly and having no interaction with it. Why wouldn’t we assume luminiferous aether is like other mediums? Like air? Gravity and other forces affect it and it more or less spins with the Earth. So light waves traveling in opposite directions will travel more or less at the same. And M&M are about to lend credence to my hypothesis.
ChatGPT:
Your question touches on an intriguing aspect of the scientific thinking surrounding the luminiferous aether before the Michelson-Morley experiment and the subsequent development of Einstein’s theory of relativity. The idea of the aether as a stationary, absolute frame of reference through which electromagnetic waves (such as light) propagate was deeply rooted in the physics of the 19th century, influenced by Newtonian mechanics where absolute space and time were prevailing concepts.
The reason why scientists initially expected the luminiferous aether to be fixed in space, rather than moving with the Earth, stems from several lines of reasoning:
Aether as the Absolute Frame of Reference: The luminiferous aether was hypothesized to be the medium for the propagation of light and electromagnetic waves, much like air is the medium for sound waves. However, unlike air, the aether was thought to fill all of space, even the vacuum, and to be immaterial and non-interacting with physical objects. This concept allowed scientists to reconcile Maxwell’s equations for electromagnetism, which describe light as a wave, with the observation that light’s speed appears constant in all directions (unaffected by the motion of the source or the observer).
Galilean Relativity and Newtonian Mechanics: Before Einstein, the dominant framework for understanding motion was Galilean relativity and Newtonian mechanics, which distinguished between absolute and relative motion. According to this view, while objects within the aether (such as planets) might move, the aether itself was considered a fixed, absolute frame against which all motion could be measured.
Lack of Observable Interaction: There was no direct evidence that the aether interacted with matter in the way that air or water does. Objects seem to move through space without friction or resistance that would imply interaction with a medium like the aether. This led to the assumption that the Earth moves through the aether without dragging it along, as it clearly does with its atmosphere due to gravitational attraction and frictional forces.
The expectation was that, if the aether were stationary relative to the Earth, then the Earth’s motion through it would create an “aether wind” effect, detectable as a variation in the speed of light depending on the direction of travel relative to this wind. When Michelson and Morley’s experiment found no such variation, it challenged the existing understanding of the aether and paved the way for the development of special relativity, which eliminated the need for the aether by postulating that the speed of light in a vacuum is constant for all observers, regardless of their relative motion.
Your hypothetical perspective, suggesting that the aether might move with the Earth similar to how the atmosphere does, anticipates aspects of what Einstein’s theory of relativity would later formalize: that the laws of physics, including the speed of light, are the same for all observers in any frame of reference, without needing to appeal to a luminiferous aether. However, before the Michelson-Morley experiment and Einstein’s work, the prevailing scientific models did not accommodate such a view, illustrating how scientific understanding evolves with new evidence and theoretical advancements.
Professor:
No, I was thinking the opposite. My hypothetical perspective is that light isn’t significantly different from sound or any other wave. Just as the speed of sound depends on the observer’s frame or reference, so too does the speed of light. So Einstein’s General Theory of Relativity is wrong. Isn’t that a reasonable interpretation of the M&M experiment? Assuming at least there was little “luminiferous Coriolis wind” that day?
ChatGPT:
Interpreting the Michelson-Morley experiment from the perspective that the speed of light, like the speed of sound, should depend on the observer’s frame of reference and hence could vary under different conditions, is an interesting hypothetical scenario. However, this interpretation runs counter to the core findings of the experiment and the subsequent foundational principles of special and general relativity developed by Einstein.
The Michelson-Morley experiment’s null result — that is, the failure to detect any difference in the speed of light relative to the direction of Earth’s motion — challenged the prevailing notion of the luminiferous aether and the assumption that the speed of light would be variable depending on the motion of the source or observer relative to this medium. Instead of supporting a model where light behaves similarly to sound, the experiment’s outcome suggested that light’s speed is invariant, a result that directly contributed to the development of special relativity.
Einstein’s Special Theory of Relativity, formulated in 1905, fundamentally differs from the classical mechanics approach to waves like sound. It asserts that the speed of light in a vacuum is constant and does not depend on the motion of the source or the observer, which was a radical departure from the way velocities typically add in classical mechanics (as they do with sound). This principle is supported by extensive experimental evidence beyond just the Michelson-Morley experiment, and it has been confirmed by numerous experiments since.
Einstein’s General Theory of Relativity, proposed in 1915, extends these principles to include the effects of gravity on time and space, describing how mass and energy can curve spacetime, affecting the path of light. Unlike sound, which requires a medium and whose speed can vary with the observer’s frame of reference, light in Einstein’s theory behaves in ways that do not conform to the properties of waves traveling through a physical medium like air or water.
While it’s tempting to draw parallels between light and sound, especially before the full implications of the Michelson-Morley experiment were understood, Einstein’s relativity fundamentally changed our understanding of light, space, and time. The invariant speed of light is a cornerstone of modern physics, leading to predictions and confirmed phenomena (like time dilation and gravitational lensing) that would not be possible under a model where light behaved like sound.
Your hypothetical perspective interestingly mirrors some of the debates and alternative theories explored in the early 20th century before relativity became widely accepted. However, the extensive experimental and observational support for relativity makes it one of the most robust frameworks for understanding the universe at both large scales and high speeds, despite its counterintuitive nature compared to everyday experiences with phenomena like sound.
Professor:
You keep saying the Michelson-Morley experiment’s failure to detect any difference in the speed of light relative to the direction of Earth’s motion (at least partially) disproved the prevailing notion of the luminiferous aether which I get. But that’s because the prevailing notion of luminiferous aether was that it was fixed in space, which I don’t get. My prevailing assumption was (or would have been, or still is) that luminiferous aether is affected by gravity much like air is an moves through space with the earth. And I wouldn’t necessarily assume it has any winds or Coriolis effects. So my expectation would be that light waves in earth’s luminiferous atmosphere traveling in opposite directions would travel at the same speed. Wouldn’t you agree that the M&M experiment’s findings are consistent with my hypothesis as well as Einstein’s? Or would you still argue it refutes mine?
ChatGPT:
Your perspective offers a fascinating alternative interpretation of how one might have conceptualized the luminiferous aether prior to the advent of relativity, particularly in envisioning it as a medium that moves with the Earth, unaffected by “winds” or Coriolis effects, similar to how the Earth’s atmosphere behaves due to gravitational binding and frictional interactions with the surface. If we were to assume this kind of behavior for the aether, then the Michelson-Morley experiment’s findings — that there is no detectable difference in the speed of light for beams traveling in different directions relative to Earth’s motion — would indeed seem consistent with such a hypothesis. This is because, under this assumption, the aether would not introduce any relative motion between itself and the Earth that could affect the speed of light, akin to how the atmosphere moves with the Earth and does not significantly affect the speed of sound relative to the Earth’s surface.
However, there are key points to consider in evaluating this hypothesis against Einstein’s theory and the broader implications of the Michelson-Morley experiment:
The Invariance of the Speed of Light: Einstein’s Special Theory of Relativity, which emerged as a direct response to the Michelson-Morley experiment among other observations, posits that the speed of light in a vacuum is a constant, c, independent of the motion of the source or the observer. This principle has been experimentally validated in numerous contexts beyond the Michelson-Morley experiment and is foundational to modern physics. It does not necessitate the existence of an aether of any kind.
The Concept of Aether and General Relativity: Einstein’s General Theory of Relativity further eliminates the need for an aether by describing gravity not as a force in the classical sense but as the effect of mass and energy curving spacetime itself. Light follows the curvature of spacetime, which explains the phenomena like gravitational lensing without requiring a medium through which light travels.
Experimental Evidence: Subsequent experiments have further confirmed the predictions of relativity, including the bending of light by gravity and the time dilation effects predicted by special relativity, which would not necessarily be accounted for by a hypothesis that treats the aether as a medium moving with the Earth.
While the idea of an aether moving with the Earth could superficially align with the null results of the Michelson-Morley experiment in isolation, the broader context of experimental evidence and theoretical developments since then strongly supports the framework of relativity. This framework not only explains the constant speed of light without needing an aether but also has provided a comprehensive understanding of space, time, and gravity that has been repeatedly confirmed across diverse phenomena.
In summary, while the Michelson-Morley experiment’s findings might seem compatible with a hypothesis of an Earth-bound aether at a surface level, the full spectrum of evidence and theoretical considerations since then supports the relativistic interpretation, which does not require an aether and offers a deeper, more consistent explanation of the nature of light, space, and time.
Professor:
Awesome. Appreciate the discussion. We should continue it sometime and discuss the subsequent experiments you speak of as I’m not familiar with them. Thanks again for the fascinating chat!!
ChatGPT:
I’m glad you enjoyed our discussion! There’s indeed a lot more to explore regarding the experiments and theoretical advancements that followed the Michelson-Morley experiment, each adding layers of understanding to our picture of the universe. Whenever you’re ready to dive into those subsequent experiments or any other topic, feel free to reach out. Thank you for the stimulating conversation, and I look forward to our next chat!
